1. Field of the Invention
The present invention relates to an optical information recording apparatus and an optical information recording medium which includes a light-condensing layer.
2. Description of the Related Art
The recent development of an information-oriented society through the optical communication has required a new communication system that provides a capability of rapidly communicating a large volume of information. As an optical device indispensable to developing such a high-volume and fast optical communication, there may be referred an optical information recording medium that condenses a high volume of optical information. Moreover, with digitization of an image such as a TV image or enhancement of an image quality such as the high-definition of a TV image, it is quite pressing to develop a high-volume optical information recording medium that may keep the quality of an image high and record the high-quality image for a long time.
Today, as the optical information recording medium, the DVD (Digital Versatile Disk) having a capacity of 4.7 GB on one side comes into practical use. It is widely utilized for a computer medium or a medium for recording a high-volume moving picture such as a video application. Practically, this kind of DVD may be used as a ROM (DVD-ROM) on which optical information is directly written as well as a rewritable recording and reproducing optical medium. The optical information recording medium has been developed for enhancing the recording density. In order to make the density of the information recording higher, a laser beam of a short wavelength is used. The laser beam has a wavelength of 650 nm that is shorter than the wavelength (780 nm) of the laser beam used for a CD or the like. However, in order to treat a high volume of information such as a computer graphics picture or a digital Hi-vision image, it is necessary to make the recording density four to five times as high as that for the foregoing laser beam of a short wavelength. To achieve it, a new optical disk is now being developed. The new optical disk is fitted into a blue semiconductor laser of a quite short wavelength (405 nm) and may have a volume of 27 GB on one side.
As another technology of enhancing the recording density, formation of a light-condensing layer may be referred. The light-condensing layer is a coating film to be formed on the top or the bottom of an optical recording layer formed on the optical recording medium. This coating film serves to reduce a beam spot of an incident ray passing through or reflected on the film itself, for making the recording density higher. The combination of this technology with the foregoing technology of shortening a laser wavelength makes it possible to increase the volume of the optical disk.
One of the mechanisms for bringing about the light-condensing effect is an absorption-saturation phenomenon. The phenomenon utilizes such a nonlinear optical characteristic that the light-condensing layer serves to pass a ray having a higher intensity than the absorption-saturation amount therethrough and absorb the ray having a lower intensity. The spatial intensity of a laser beam used for reading or writing information is distributed in the Gaussian manner. Hence, when the beam is passed through the light-condensing layer, the base portion of the beam of a lower intensity is absorbed by the light-condensing layer, while the central portion of the beam of a higher intensity is passed through the layer. This makes it possible to reduce the beam in diameter after passed through the layer.
Today, as a component material of this kind of layer, a phthalocyanine system organic film or a chalcogenide system compound may be referred as described in JP-A-8-96412. Also, it is known that the organic material such as a thermochromic material as described in JP-A-6-162564 or a photochromic material as described in JP-A-6-267078 may be used for composing the light-condensing layer.
However, the light-condensing material such as the thermochromic material or the photochromic material raises its extinction coefficient by the heat or the excitation caused by beam radiation, so that the layer composed of such a material may lower its transmittance or reflectance. The resulting laser beam may not reach the required power for reading information from the medium. In order to overcome this shortcoming, in JP-A-2001-273679, an interference layer is laminated on the beam-incident side of a super resolution reproducing film (composed of a light-condensing material) whose extinction coefficient is selectively made higher by the greater beam radiation than the predetermined threshold volume. The interference layer is composed of a layer of a low refractive index and a layer of a high refractive layer. The interference layer serves to increase the reflectance of the beam radiated onto the optical information medium.
The JP-A-2000-34381 describes the optical recording medium having a mask layer whose light transmittance characteristic is varied by the beam radiation. This mask layer is a light-condensing function layer. The JP-A-10-340482 describes a recording medium having a glass layer in which the intensity distributions of the radiated beam and the transmitted beam are varied nonlinearly, for the purpose of improving endurance of an organic film considered as a super resolution film. The super resolution film used herein is a light-condensing function layer.